Quick Query Checklist — Take 5 minutes before you send out your query to make absolutely sure you have everything you need in it!

Follow the Sub Guidelines — Exactly what it says. So many querying authors shoot themselves in the foot by not following the submission guidelines. Follow directions, and you’ll put yourself ahead of 90% of querying authors.
[Heartily endorsed by literary agents]

POP QUIZ! Do You Know Your Query Etiquette?Round1 and Round 2 — *announcer voice* Do you have what it takes to survive the query trenches? STEP RIGHT UP AND FIND OUT!
[Endorsed and taken by literary agents — also contains occasional agent commentary]

Handling Rejection in a Brutal Business — Every author faces rejection. That is just a fact of the industry and of being an artist in general. But rejection can also be channeled into positive progress.
[This thread has also been endorsed and quoted by literary agents. If you like gif posts better, try Keep Moving Forward.]

Italicized Thoughts vs Direct Narration — One of my biggest baby author sins was throwing in excessive italicized thoughts from the MC’s POV instead of smoothly integrating them into the narrative. This goes through why the latter can be more effective and provides examples of both.
[And, you guessed it, it has also been endorsed by literary agents.]

Filtering and Distancing Language— Don’t put a barrier between your main character and your reader if you’re trying to write close 1st person or 3rd person! Filters and distancing language are easy to remove (and cut down on your word count!), but make an incredible amount of difference.
[Yet again, endorsed by literary agents as well as Big Five editors.]

Don’t Be a Dick (especially to assistants) — Assistants do a LOT of the heavy lifting in publishing. Don’t be a dick to them. Seriously.
[Do you even need to guess with this one? YEP. Agents and Big 5 editors and — surprise! — MANY assistants to both]

Historical Research 101 — A basic guideline to doing historical research for books: Rae edition.
[Endorsed by fantastic histfic writers!]